The present invention relates to anti-skid devices and especially to an anti-skid brake for vehicle wheels which utilizes the road-wheel torque to power the wheel brake so that the resisting torque applied by the brake cannot exceed the torque generated by the road on the wheel of the vehicle.
It has been known for some time that, under most road conditions, skidding does not occur suddenly as a result of an instantaneous switch from the coefficient of rolling friction to the coefficient of sliding friction at the wheel-road contact and a simultaneous switch in the brake from a coefficient of sliding friction to a coefficient of static friction. The change from rolling contact to skidding is a gradual, but rapid transition where the coefficient of friction varies with slippage while the brake switches from sliding friction to static friction at the instant the wheel reaches 100% slippage.
Many anti-skid devices sense wheel rotation, vehicle acceleration, or brake pressure to detect a skid condition. Even though some prior art devices measure torque to detect slippage, most of these systems use electronic circuits to pump or pulse the brake pressure causing the tire-road contact to alternate between 100% skidding and free rolling conditions. These prior art systems improve control during braking, but have a number of disadvantages including: (a) not providing optimum stopping conditions, but instead increase stopping distance by operating in the high percentage skid and free roll areas, thus preventing only sustained full skids; (b) their effectiveness is influenced by road conditions since any given wheel rotation rate, vehicle acceleration, or brake pressure could occur at any point in the range of skid; and finally, (c) the systems are complicated by complex electronic-hydraulic circuitry resulting in high manufacturing, installation and maintenance costs in addition to low reliability.
There are also a number of anti-skid devices which sense wheel-road torque and use this to operate servo valves. These similarly have disadvantages such as the brake pressure not being limited at the optimum stopping condition, but merely modified at the 100% skid area, thus permitting possible 100% skids.
The present invention, on the other hand, relates to a combination of devices which are directed toward preventing skidding of a vehicle on any road by adapting the braking force to the existing road wheel friction condition. Thus, the invention will sense the torque applied by the road to the wheel and use this wheel torque to power and control the brake, thus providing a limit on the braking capacity which is dependent on the existing road conditions.
By using the difference in pressure between the pilot brake and the main brake to control and power the brake action, the need for a servo valve is eliminated. The present invention is an improvement over the invention in my prior U.S. Pat. Nos. 3,872,952 and 3,923,345.